1. Field of the Invention
The present invention relates to multilayer ceramic substrates, methods for making the same, and composite green sheets for making multilayer ceramic substrates, and in particular, to a multilayer ceramic substrate produced by the application of a zero shrinkage process, a method for making the same, and a composite green sheet for making a multilayer ceramic substrate.
2. Description of the Related Art
An example of a multilayer ceramic substrate related to the present invention is described in Japanese Unexamined Patent Application Publication No. 2000-25157 (Patent Document 1). Patent document 1 discloses a composite laminate and a production method therefor using a zero shrinkage process. In particular, patent document 1 discloses a composite laminate that can be produced while inhibiting shrinkage caused by baking and that can be used in an as-baked state, and a production method therefor. As a preferable example, a multilayer ceramic substrate having a structure described below and a production method therefor are disclosed in patent document 1.
That is, the multilayer ceramic substrate includes a base material layer including an aggregate of first particles containing a glass material and a first ceramic material, and a constraining layer including an aggregate of second particles containing a second ceramic material that does not sinter at a temperature at which the glass material melts. At least a portion of the first particles is in a sintered state. In contrast, the second particles are in an unsintered state but are bonded to each other since a portion of the first particles containing the glass material diffuses or flows into the constraining layer.
In order to prepare such a multilayer ceramic substrate, a green laminate including a base material layer in a green state containing the first particles and a constraining layer in a green state containing the second particles is prepared, and then the green laminate is baked. During the baking step, at least a portion of the first particles is sintered. Furthermore, in the baking step, a portion of the first particles, typically a portion of the glass material contained in the first particles, diffuses or flows into the constraining layer. As a result, although the second particles do not sinter, the second particles are bonded to each other via the portion of the first particles, in particular, via the glass material.
According to the production method described above, the second particles do not sinter during the baking step. Thus, the constraining layer containing the second particles inhibits shrinkage of the base material layer, and shrinkage of the multilayer ceramic substrate caused by baking is inhibited as a whole. As a result, dimensional variations among the resulting multilayer ceramic substrates are reduced. Moreover, there is no need to subsequently remove the constraining layer from the resulting multilayer ceramic substrate since the second particles contained in the constraining layer are bonded to each other while a portion of the first particles containing the glass material diffuses or flows into the constraining layer.
However, when the technique described in Patent Document 1 described above is used, there is a problem in that, depending on the type of the glass material contained in the base material layer, the glass material does not sufficiently permeate into the constraining layer. The main cause of this problem is presumably the high viscosity and low flowability of the glass material during melting.
A first conceivable solution to this problem is to use a glass material having low viscosity in the base material layer. However, when a glass material having low viscosity is used and, for example, formation of a capacitance in the base material layer is attempted, there is a problem in that necessary capacitance cannot be obtained due to excessively low relative dielectric constant of the base material layer. In other words, selecting a glass material having low viscosity and satisfying the required electrical characteristics or adjusting the composition of the glass material is technically difficult.
Another conceivable approach for solving the above-described problem of the glass material not sufficiently permeating into the constraining layer is to reduce the thickness of the constraining layer. However, when the thickness of the constraining layer is reduced, the shrinkage-inhibiting effect of the constraining layer is reduced. As a result, a problem of warpage in the resulting multilayer ceramic substrate occurs.
In particular, when the glass material contained in the base material layer is a crystallized glass material, this problem is more serious since the crystallized glass inherently has high viscosity.